imsettings: add xfconf dependency and xfce-layer to LAYERDEPENDS

* this isn't optimal, but this issue was ignored by recipe author for too long:
  http://lists.openembedded.org/pipermail/openembedded-devel/2015-July/102253.html
  http://patchwork.openembedded.org/patch/86091/



Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>